FIG. 1 is a structural view showing a conventional narrow bandwidth laser shown, for example, in a magazine called "CAN. J. PHYS. VOL 63 ('85) 214".
This FIG. shows a laser medium 1, a full reflection mirror 2, an incomplete reflection mirror 3, an etalon 4 for rough tuning, an etalon 5 for fine tuning and a laser beam 6.
A brief description of the operation of this laser follows. In FIG. 1 laser medium 1 is surrounded by a light resonator consisting of the full reflection mirror 2 and the incomplete reflection mirror 3, whereby light is amplified while being reflected within the light resonator numerous times before exiting as laser beam 6. Some laser resonators found in, for example, excimer lasers, semiconductor lasers, pigment lasers and some types of solid-state lasers, have large oscillating wavelengths. By inserting spectroscopy elements into the light resonator, their oscillating wavelength width can be narrowed. For example, a laser beam extremely close to monocolor can be obtained by using a plurality of Fabry-Perot etalons (hereinafter to be abbreviated as etalon).
In the example of FIG. 1, two etalons, that is, the etalon 4 for rough tuning and the etalon 5 for fine tuning are inserted into the light resonator. FIG. 2 shows various wavelength profiles describing the principle behind the narrowing of the oscillation width of the laser. FIG. 2(a) shows a spectroscopy characteristic of the etalon for rough tuning. The peak position .lambda.m.sub.1 of the spectroscopy characteristic is represented by the following equation (1), ##EQU1##
Here, n is the index of refraction of a material existing between two mirrors forming the etalon, d is a distance between the mirrors, .theta..sub.1 is an angle when light is incident upon the etalon, and m is an integer. Peaks correspond to the different of value of m. As is clear from equation (1), peak wavelength of the mountain can be changed arbitrarily by changing the value of any of n, d, and .theta.. The distance between peaks is called free spectral range (hereinafter to be abbreviated as FSR), and is represented by the following equation (2). ##EQU2## The half band width .DELTA..lambda., of each peak is represented by the following equation (3). ##EQU3## Here F.sub.1 is called finesse and is determined by the performance characteristics of the etalon.
FIG. 2(c) shows the spectroscopy characteristic of the gain of a laser medium. When spectroscopy elements do not exist in the light resonator to narrow the wavelength of the light, light is amplified to become a laser beam over the entire wavelength in the range of the gain. FIG. 2(a) illustrates the state where loss is minimized at only the position of .lambda..sub.0 due to the existence of the etalon for rough tuning. Therefore light is amplified and oscillated at only the vicinity of this wavelength, by deciding d.sub.1 and the like so that the peak position .lambda.m.sub.1 of the talon for rough tuning is equal to any wavelength .lambda..sub.0 in the range where gain exists, and the peaks other than .lambda.m.sub.1 do not come into the wavelength where gain exists.
The minimum value of FSR.sub.1 is determined when there is only one peak and finesse F is determined by the performance characteristics of the etalon. Since the finesse value is about 20, there is a limit to wavelength width which can be narrowed only by one etalon for rough tuning.
According to the present invention, another etalon for fine tuning 5 is used. A spectroscopy characteristic of the fine tuning etalon, for example, is illustrated in FIG. 2(b). Therein, the peak wavelength .lambda.m.sub.2 should be .lambda..sub.0 and FSR.sub.2 should be FSR.sub.2 .gtoreq..DELTA..lambda..sub.1. When the wavelength to be amplified and oscillated is desired to be narrower, another etalon can be used.
According to the invention, the laser beam, whose spectroscopy characteristic was, for example, that illustrated in FIG. 2(c), is to oscillate only in a narrow range including .lambda..sub.0 where each peak of the etalons overlap each other as a center as shown in FIG. 2(d). Actually, laser beams pass through the etalons numerous times during oscillation, whereby the wavelength width of the laser beam becomes 1/2-1/10 of the wavelength determined by two etalons.
In the way above mentioned, the wavelength of the laser beam can be narrowed as described in the aforesaid magazine, and stability in a short duration can be achieved by improving the light resonator and making the incident angle .theta. small. However, stability in a long duration will not occur due to thermal problems, such as wavelength shift due to generation of heat several when the laser beam passes through the etalons. This problem is explained with reference to FIG. 3.
FIG. 3(a) illustrates an enlarged spectroscopy characteristic of the etalon for rough tuning, wherein the solid line shows the spectroscopy characteristic immediately after oscillation. Generation of heat after oscillation cause the etalons to deform. This deformation does not degrade the characteristics of etalons, but it changes the gap length of etalons and as a result shifts the wavelength. Equation (4) shows the between the shift quantity and the change of d due to the deformation of the etalons. ##EQU4## The direction of the wavelength shift is determined by the structure of the etalon and the like, and wavelength shifts in a certain direction due to the generation of heat by the laser beam occur when a specified etalon is used. The state of shift at that time is shown by the broken line function in FIG. 3(a). The etalon for fine tuning also has a similar wavelength shift as shown in FIG. 3(b). The shift quantity of the etalon for fine tuning becomes smaller by the quantity which is the difference between the etalon distance d.sub.2 and the etalon d.sub.1 when d.sub.2 is bigger than d.sub.1.
The problem at that time is that the peak wavelengths .lambda.m.sub.1 and .lambda.m.sub.2 of spectroscopy characteristic of the two etalons deviate. Light transmission quantity when the two wavelengths overlap is reduced compared with the case where .lambda.m.sub.1 =.lambda.m.sub.2. The state of laser oscillation at that time is shown in FIG. 3(c). After a long oscillation, the laser output wavelength-shifts from .lambda.m.sub.1 =.lambda.m.sub.2 and the output is reduced. When the shift quantity is large, another mode oscillation other than the etalon for fine tuning can occur.
Conventional narrow band width laser devices do not have means for compensating the wavelength shift due to thermal problems of the etalons nor do they have means for stopping output reduction which occurs when two etalons are used. Therefore, it has a problem that it can only be applied to a low output laser whose effect of thermal deformation is small.